Mag-Drive/Seal less vs. Canned Motor Pumps 3

[pumped]

I'm in need of some assistance. We've been having trouble with our canned motor pumps for the past number of years.

Possible solutions at this point are mag-drive pumps, seal less pumps or displacement chambers. We've replaced the same canned pumps on another system with displacement chambers and with great succes since the 70's (much dirtier system than what we currently pump and slightly different failures than what we currently see). In order to do it on our current system, we need to use a very clean gas and at a specified pressure.

The pumped fluid is lethal but not terribly corrosive. We can't have mechanically sealed pumps due to the lethal nature of the fluid. Sealed or seal/less is a must. The required flow rate downstream is 25 USgpm at a TDH of 130ft (low flow, high head). We have sufficient NPSHa and the system is gravity fed. The pumps seem to cavitate which causes the impeller to hit the front bearing housing and we've also seen the rotor assembly contact the wear surface on the bearing. This generates heat which causes the fluid to vaporize and plate out on the bearing. When this happpens, the pump fails. All of this happens over a matter of seconds. Not sure if process measurement devices would help us in such a case. We have a glycol cooling jacket around the pump as well as glycol on the backflush piping coming into the rear end of the pump. The stator is oil filled to assist in temperature dissipation. A lot of cooling to say the least. The fluid is very temperamental in terms of temperature variation.

In order to acquire a successful prime, lately we've been bumping the pump about 5 times in 30 second intervals to ensure that the pump is flooded completely (suction, discharge and backflush). I don't have a great deal of experience with pumps. Just learning the ropes.

Looking for any help or advice that someone could pass on (i.e. pros and cons on mag drive vs. canned vs. mechanically sealed, displacement chambers, proper material selection and so forth).

Thanks,

TP

 

[Newkiwi]

If you want to consider a diaphragm pump, look at

http://www.hydra-cell.com/.

It will require a relieving valve to the pump suction if it is possible block the discharge. Although diaphragms occasionally fail it seems this could be controlled by replacing the diaphragms before the expected failure time. At least you should be able to get a much higher service life than you have been experiencing with mag drive pumps in this application. I've never used this pump brand but had considered them for a different high head, low flow application. The main advantage of the design is that there is minimal pulsation which is charactaristic of most diaphragm designs.